My parents live on a subdivided lot and the letter at the end is used as a decimal component of the house number:
Neighbors up the road have 1234A and my parents have 1234B.
When I moved to Germany I was staying in an AirBNB. The instructions were to just "Ring [Name on doorbell]". When I asked what the apartment or flat number was, it was an alien concept---the host just gave me the address again.
There may also be an addition of multiple letters. For example: in the Netherlands it is not uncommon to see a number like '45 bis' or even '45 bis B'.
'bis' is latin for 'twice', indicating that the building was split into 2 separate houses at some point in time.